In Situ Sampling of Relative Dust Devil Particle Loads and Their Vertical Grain Size Distributions.

During a field campaign in the Sahara Desert in southern Morocco, spring 2012, we sampled the vertical grain size distribution of two active dust devils that exhibited different dimensions and intensities. With these in situ samples of grains in the vortices, it was possible to derive detailed vertical grain size distributions and measurements of the lifted relative particle load. Measurements of the two dust devils show that the majority of all lifted particles were only lifted within the first meter (similar to 46.5% and similar to 61% of all particles; similar to 76.5wt % and similar to 89wt % of the relative particle load). Furthermore, similar to 69% and similar to 82% of all lifted sand grains occurred in the first meter of the dust devils, indicating the occurrence of "sand skirts." Both sampled dust devils were relatively small (similar to 15 m and similar to 4-5 m in diameter) compared to dust devils in surrounding regions; nevertheless, measurements show that similar to 58.5% to 73.5% of all lifted particles were small enough to go into suspension (<31 mu m, depending on the used grain size classification). This relatively high amount represents only similar to 0.05 to 0.15wt % of the lifted particle load. Larger dust devils probably entrain larger amounts of fine-grained material into the atmosphere, which can have an influence on the climate. Furthermore, our results indicate that the composition of the surface, on which the dust devils evolved, also had an influence on the particle load composition of the dust devil vortices. The internal particle load structure of both sampled dust devils was comparable related to their vertical grain size distribution and relative particle load, although both dust devils differed in their dimensions and intensities. A general trend of decreasing grain sizes with height was also detected.